11 June 2014

New friction stir welding tool also functions as a temperature sensor for improved quality. Click to enlarge.

Researchers at University West in Trollhättan have addressed two drawbacks to the robotic friction stir welding (FSW) joining process for mixed materials—path accuracy and temperature—with the development of a deflection model and integral temperature controller.

Car manufacturers are increasingly looking to a hybrid or mixed materials design, in which a combination of different materials such as steel and aluminium are joined, for weight reductions in their vehicles. With classic welding methods, joining of dissimilar materials has not been possible. With friction stir welding (FSW), on the other hand, high quality dissimilar joints can be obtained. (Earlier post.) The use of industrial robots also allows FSW of materials along complex joint lines.

Source: Jeroen de Backer.

In FSW, a rotating non-consumable cylinder is pressed into the material. The combination of frictional heat and the mechanical “stirring” creates a high-quality welding joint, without melting the material. The welding temperature is kept below the melting point, which means that the alloy properties are not destroyed and strong joints are achieved. There have been two practical challenges with this method:

Robot compliance. This results in vibrations and insufficient path accuracy. For FSW, path accuracy is important as it can cause the welding tool to miss the joint line and thereby cause welding defects.

Variable heat dissipation on complex geometries. Variable heat dissipation in the workpiece causes great variations in the welding temperature. Especially for force-controlled robots, this can lead to severe welding defects, fixture- and machine damage when the material overheats.

To address the first issue, the researchers first measured path deviations post-weld and later by using a camera and laser distance sensor to measure deviations online. Based on that knowledge, they created a robot deflection model. The model is able to estimate the tool offset during welding, based on the location and measured tool forces. This model can be used for online path compensation, improving path accuracy and reduced welding defects.

To address the second issue, they developed a new temperature method which measures the temperature at the interface of the tool and the workpiece, based on the thermo-electric effect. The temperature information is used as input to a closed-loop temperature controller. This modifies primarily the rotational speed of the tool and secondarily the axial force.

The controller is able to maintain a stable welding temperature and thereby improve the quality and allow joining of geometries which were impossible to weld without temperature control.

The resulting robot welds with higher precision and with the temperature controller it only takes a few hours to program 3D joints, said Dr. Jeroen De Backer, who wrote his thesis on this new method. Manual programming of a similar component took up to a week.

With the aid of the robot, and the temperature measurement, the researchers have also been able to weld advanced three-dimensional joints. This enables the welding of small and more complex components with curved surfaces. Furthermore, the energy consumption of FSW is lower than when using conventional welding methods.

The research project at University West was initiated through a collaboration between Volvo Aero, SAAB Automobile and the welding equipment company ESAB.

Jeroen De Backer notes that a possible application can be hybrid and electric vehicles:

Car manufacturers aim to reduce the weight of the electric vehicle and positioning of the heavy batteries is a key factor in this. The battery consists of different metals such as aluminium and copper. Friction stir welding provides the possibility to join those materials and allows thereby integration of the battery in the vehicle chassis so that the battery becomes a part of the bearing structure.

Comments

FSW is one of those technologies that has disappointed the faithful (me included) more than once. The last big "change the world" project for FSW was the ill-fated Eclipse Very Light Jet. FSW was supposed to be the manufacturing solution that would beat the pants off of traditional riveting and welding methods, so much so that the aircraft could be manufactured for 30-50% less than its traditional counterparts. Like most technologies, success generally comes in fits and starts, so perhaps the time for FSW has come. More extensive use of Aluminum might also spur that event.

FSW is already reality for shipbuilding and the formation of railroad axle-wheel assembles. Apparently not for aviation, as graphite and resin composites will rule the day. Unlike traditional welding, the weldment throat configuration, melt rate, weld cross-section, bead, and annealing can be controlled in a myriad of ways. We're not looking at elaborate shapes, but better ways to connect standard sheets, and thinner ones. Brazing can be eliminated.

Maybe much spot welding too, which the auto industry is crazy about. I had to explain to an angry friend of mine why he couldn't sue GM when a bumper fell off on his -- spot welds are not reliable, and there is no implied warranty tthat they are stronger than the car or assembly line workers were negligent.